Pill bugs, often called roly-polies, are common, harmless crustaceans found in many gardens and damp environments. These land-dwelling relatives of crabs and shrimp often prompt questions about their size. Understanding their typical dimensions and the biological processes that influence their growth provides insight into these creatures.
Average Pill Bug Size
An adult pill bug typically measures between 0.25 to 0.75 inches (approximately 6 to 19 millimeters) in length. The species Armadillidium vulgare can reach up to 18 millimeters. Despite their armored appearance, pill bugs remain relatively small throughout their lifespan.
Factors Affecting Pill Bug Size
Several environmental and biological elements influence the ultimate size a pill bug achieves. Older individuals generally exhibit larger body sizes. Access to sufficient and quality food, primarily decaying organic matter, directly supports growth. Adequate moisture and high humidity are also necessary for their survival and development, as these land crustaceans possess gill-like structures that require continuous moisture to function. Extreme temperatures can hinder their growth, with optimal conditions allowing for healthier individuals. Variations exist among different species, some of which may naturally be slightly larger or smaller.
How Pill Bugs Grow
Pill bugs grow by molting, or ecdysis, where they shed their rigid outer exoskeleton. This process occurs in two distinct stages to reduce vulnerability. First, the back half of the exoskeleton is shed, followed by the front half a few days later. This two-part molting allows them to gradually increase in size while minimizing the time their soft, new cuticle is exposed. Pill bugs continue to molt throughout their lives, even as adults, though less frequently than during earlier developmental stages.
Pill Bugs vs. Similar Critters
Pill bugs are often confused with other small creatures, particularly sow bugs and millipedes, due to their similar appearances and habitats. Sow bugs are closely related crustaceans that share a similar size range with pill bugs. A key distinction is that sow bugs cannot roll completely into a tight ball when disturbed, and they possess two noticeable tail-like appendages, called uropods, at their rear. Pill bugs, in contrast, lack these prominent uropods and can fully encapsulate themselves into a spherical shape.
Millipedes are distinct from pill bugs; they are arthropods but not crustaceans. Millipedes are typically much longer and have a more elongated, cylindrical body with many segments, each bearing two pairs of legs. Unlike pill bugs, which have seven pairs of legs, millipedes possess a greater number of legs, giving them a worm-like appearance. While some millipedes can coil up when disturbed, they generally do not form a tight, perfect ball like a pill bug.